The invention relates to alumina-based ceramic grains obtained by fusion.
Aluminous ceramic grains are useful, among other applications, for the manufacture of abrasive tools. In general, abrasive tools are classified according to the method of forming the ceramic grains of which they are composed: free abrasives (grains used by spraying them or used in suspension, with no backing); coated abrasives (a cloth or paper type backing coated with the grains, these being conventionally arranged in several layers); and bonded abrasives (bonded grains in the form of grinding wheels, sticks, etc.).
In the case of bonded abrasive tools, the abrasive grains are pressed with an organic or glassy binder. Glassy binders generally consist of oxides, essentially silicates. The bonded grains must themselves exhibit good mechanical abrasion properties, in particular must have good toughness. They must also be able to be bonded strongly to the binder (interfacial strength).
At the present time, there are various families of ceramic grains that allow all these applications to be covered with a variety of performance. In particular, two large families can be distinguished in which the grains are obtained by a sol-gel method or by fusion.
The sol-gel method, as described for example in EP 1 228 018 (U.S. Pat. No. 6,287,353), makes it possible to manufacture grains with a very fine crystalline structure, conventionally on a submicron scale, which gives them excellent cutting effectiveness and a long life time. However, the productivity of the sol-gel method is low and incurs high manufacturing costs.
Fused grains, obtained by fusing the raw materials or “fused grains”, conventionally have much coarser crystalline structures and have a lower cutting effectiveness and a shorter life time. Fused grains containing mainly alumina are described, for example, in U.S. Pat. No. 4,157,898. The main advantage of these grains is their low manufacturing cost.
The composition of the grains is important, but the manufacturing process also plays a determining role on the performance. Thus, for a given composition, a microstructure obtained by the sol-gel route, and offering advantageous properties, cannot easily be obtained by fusion.
Table 1 below provides, for comparison, the results in a fracturing resistance test (test A), described in greater detail later on in the description, for two high-alumina abrasive grains of the prior art. These two grains are manufactured and sold by Saint-Gobain Industrial Ceramics. The white corundum grain is obtained by fusion and the Cerpass grain by the sol-gel method. As table 1 shows, the chemical compositions are very similar (the balance is alumina). However, white corundum gives a 119% result in test A, whereas Cerpass gives 375%.
TABLE 1TestSiO2TiO2Na2OMgOCaOFe2O3Cr2O3AWhite<0.1%<0.05%0.27%<0.02%<0.02%0.02%—119corundumCerpass0.061%0.096%<0.03%0.009%0.014%—0.003%375
FIGS. 1 and 2 appended hereto show, in cross section, white corundum and Cerpass grains, respectively.
There is therefore a need for fused aluminous grains offering better performance both in terms of life time and cutting effectiveness than that of the current fused aluminous grains, but which can be manufactured for a substantially lower cost than that of the aluminous grains obtained by the sol-gel method.